Is unsupervised learning a branch of AI?
There is a problem with confining Artificial Intelligence to a single definition, because it has become an umbrella term encompassing many fields of science. It has come a long way from the "thinking machines" of the 50s. Actually, the term was coined in a summer workshop in 1956, whose proposal was:


  The study is to proceed on the basis of the conjecture that every aspect of learning or any other feature of intelligence can in principle be so precisely described that a machine can be made to simulate it.


So even from its very first introduction the field of AI included learning. Personally, I've never seen a definition of AI, that doesn't include learning, of which unsupervised learning is clearly a part. So by any means clustering algorithms, which are unsupervised learning algorithms, can be considered AI. This can be confirmed in multiple sources, e.g. wikipedia.